A showdown is nearing in the Senate on whether to allow drilling in the Arctic National Wildlife Refuge, one of the Bush administration's top domestic priorities, and both sides say the outcome could hinge on a single vote.

Senate Republicans said Tuesday they fully anticipate a provision on developing oil in the refuge, known as ANWR, to be included as part of a budget measure to be take up by the full Senate next week.

The latest tally shows 49 senators ready to vote for the pro-drilling provision when it gets to the Senate floor, one shy of the number needed to overcome strong Democratic opposition, according to GOP sources.

Pro-drilling forces in the Senate distributed a memo among many of the GOP offices on Tuesday that urged an all-out push to try to find one more senator that might be persuaded to shift allegiance and join the pro-drilling ranks.

Dick Cheney has been working madly to secure the 50th (vote),'' said the e-mail sent to GOP offices.

A spokeswoman for Cheney, Jennifer Millerwise, dismissed the reference to Cheney and said the vice president at this time was not actively engaged in lobbying on the issue, although he continues to strongly support developing the refuge's oil.

He's not contacted any (senators) on this issue,'' she said.

Senate Majority Leader Bill Frist of Tennessee is anticipating that a provision on ANWR will be included by the Senate Budget Committee when it finishes its budget resolution on Thursday, according to his spokesman, Bill Hoagland.

Development of the millions of barrels of oil believed to be under the coastal plain of the refuge in the far northeastern corner of Alaska has been at the heart of President Bush's energy agenda.

But many leading Democrats, including several who hope to challenge Bush in the 2002 election, are determined to protect the 100-mile-long sliver of tundra where ANWR's oil is located. Such protection has been an obsession for environmentalists who insist that drilling will destroy its value as a sanctuary for polar bears, musk oxen, caribou and migratory birds.

Bush has argued the refuge's oil anywhere from 5.6 billion to 16 billion barrels should be tapped to reduce America's dependance on foreign crude, although the refuge oil won't to available for three to four years even if Congress gives the go-ahead.

With gasoline prices averaging $1.71 cents a gallon this week and topping $2 in many places, and people worried about oil supply problems if war erupts in Iraq, pro-drilling forces in the ANWR debate believe they have the public's attention as never before.

On Tuesday, Interior Secretary Gale Norton called a half dozen groups, including farming and union interests, to her office and urged them to go to Capitol Hill and knock on doors and help sell the message'' on Arctic drilling to any fence-sitting senators.

Appearing before a House committee Wednesday, she laid out arguments on the need for ANWR's oil as a matter of national security, insisting it can be pumped while protecting the environmental values we all hold dear.'' She disputed environmentalists' characterization of the coastal plain as ecologically critical, describing it during winter as an area of flat nothingness.''

The House approved a refuge drilling bill in the last Congress, but it was stymied in the Senate. Drilling supporters say if the Senate acts, they are confident the GOP-controlled House will go along with the Senate measure.

But Democrats, led by presidential aspirants Sens. Joe Lieberman of Connecticut and John Kerry of Massachusetts, vowed to filibuster any drilling proposal, meaning supporters had needed 60 votes in the past to get the measure passed.

Not so this time.

By including the ANWR provision as part of a budget resolution that is not subject to filibuster, drilling opponents would have to get 51 votes to remove it. As of Tuesday, they have just that many votes, according to both GOP and Democratic sources.

A spokeswoman for Coleman, who succeeded the late Paul Wellstone, a strong critic of drilling in the refuge, said she could not comment on Coleman's views on the refuge. Spokesmen for the other three could not be reached Tuesday evening, but none is believed to have shifted allegiance, according to a Democratic source.

Five Democrats Sens. John Breaux and Mary Landrieu of Louisiana, Sens. Daniel Akaka and Daniel Inouye of Hawaii, and Sen. Zell Miller of Georgia continue to be solid in the pro-drilling camp, as they were in the last Congress.

Republican Sens. Lincoln Chafee of Rhode Island, Susan Collins and Olympia Snowe of Maine, Mike DeWine of Ohio, Peter Fitzgerald of Illinois and John McCain of Arizona wrote a letter to Frist strongly objecting to the ANWR issue being intertwined in the budget process. The six were among eight GOP senators who voted against drilling last year and have not changed their views.